For example, in a video camera which is developed aiming at making it compact and light in weight, it is difficult to adopt, e.g. the conventional mechanical means as a stop mechanism for performing the adjustment of light quantity in taking a picture. Thus, it has been proposed to adopt an electrical dimmer device which uses, e.g. a liquid crystal as a light adjustment means instead of such a mechanical stop mechanism (see Patent Gazette of Laying-Open H11-326894).
Specifically, in the invention disclosed by the laid-open Patent Gazette, a polarizing plate is moved in and out of a light path where a liquid crystal cell is disposed to perform an initial light adjustment. Further, by controlling the potential impressed on the liquid crystal cell to make the polarized plane of the liquid crystal cell different from the polarized plane of the polarizing plate, more transmissive light quantity is attenuated. This makes it possible to perform satisfactory light adjustment with a simpler structure than the conventional mechanical stop mechanism.
However, with the above-mentioned structure, the liquid crystal cell always exists in the light path, so that the light transmissivity of the liquid crystal cell becomes a problem. Alternatively, it is conceived to provide a mechanism in which the liquid crystal cell is also moved in and out of the light path in same way as the polarizing plate. However, the arrangement of liquid crystal cell involves supply means (wiring) of a control voltage and the like, so that the mechanism for operating the whole system becomes complicated. This is opposed to the object of the apparatus, which makes it compact and light in weight.
Thus, the present applicant previously developed the liquid crystal cell whose light transmissivity is made extremely high and applied for patent on a light adjustment device which adopts this liquid crystal cell (Patent Application No. H11-322186). The invention of this earlier application uses a guest-host type liquid crystal for the liquid crystal and uses a negative type liquid crystal for its host material to make negative of the anisotropy of dielectric constant. The invention also uses a positive type or a negative type of dichromatic dye molecule for its guest material, thereby enabling the light transmissivity of 75% or more to be made, for example, when no control voltage is impressed.
However, such liquid crystal cell as just described above has heretofore had a slow response speed when the light transmissivity is changed, so that it is difficult to attain the response time, for example, equal to or less than one field period of time (e.g. 16.7 ms for the NTSC system of interlace scanning) which is required for the stop mechanism of video camera. To be specific, FIGS. 11A and 11B show response characteristics of the light transmissivity actually measured when the control voltage on the positive type liquid crystal is varied. FIG. 11A shows a state when the light transmissivity is raised. FIG. 11B shows a state when the light transmissivity is lowered.
In FIGS. 11A and 11B, the control voltage is a rectangular wave of one kHz, one cycle of which corresponds to one ms. Therefore it takes 22.5 ms for the light transmissivity to reach a target value in FIG. 11A, whereas it takes 9.9 ms to reach that value in FIG. 11B. It can be seen here that, when lowering the light transmissivity as in FIG. 11B, the response time of one field period of time or less can be attained, but when raising the light transmissivity as in FIG. 11A, it takes more time than one field period of time (e.g. 16.7 ms).
Moreover, the response speed of liquid crystal cell tends to sustain the influence of an environmental temperature. In other words, the response speed of liquid crystal cell becomes very slow particularly in low temperature (when the environmental temperature is 10° C. or under), for example, as seen in a table of FIG. 12A and a graph of FIG. 12B. On the other hand, it is assumed that the video camera and the like is used under the environmental temperature ranging from summer to winter. For example, when it is used outdoors in winter, the environmental temperature may become −10° C. or under depending on circumstances.